Method for manufacturing ceramic substrate and ceramic substrate

ABSTRACT

When a ceramic substrate is manufactured through a constraint firing step that uses a constraining layer, the constraining layer is removed without causing significant damage to a sintered base layer or an electrode formed on the surface of the sintered base layer, and the electrode can be reliably exposed. A green stacked body having a base layer and a constraining layer disposed so as to be in contact with at least one principal surface of the base layer is formed. A fired stacked body having a sintered base layer and a green constraining layer is then obtained by firing the green stacked body to sinter the base layer. Subsequently, the constraining layer is removed from the sintered base layer by vibrating media that are disposed so as to be in contact with the constraining layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a ceramicsubstrate and a ceramic substrate. In particular, the present inventionrelates to a method for manufacturing a ceramic substrate through aconstraint firing step in which a constraining layer is disposed on abase layer that is to be a ceramic substrate after firing and firing isconducted while the shrinkage of the base layer in a planar direction issuppressed, and a ceramic substrate manufactured by the method.

2. Description of the Related Art

In a ceramic substrate that must have high planar dimensional accuracyamong ceramic electronic components, firing shrinkage in a planardirection in a firing step or a variation of the shrinkage significantlyaffects product quality.

To fire a green base layer (ceramic stacked body) that is used to definea ceramic substrate (multilayer wiring substrate) after firing whilesuppressing the shrinkage in a firing step, a method for performingfiring so as not to cause firing shrinkage in a planar direction hasbeen proposed. This is achieved by performing firing (hereinafterreferred to as “constraint firing”) while constraining layers 52 a and52 b primarily composed of a sintering resistant material, such asalumina, that is not substantially sintered at a firing temperature of abase layer 51 are disposed so as to be in contact with both principalsurfaces of the base layer 51 as schematically shown in FIG. 9 (see, forexample, Japanese Unexamined Patent Application Publication No.2002-198646).

In the method of Japanese Unexamined Patent Application Publication No.2002-198646, the constraining layers that remain that are not sinteredafter the firing step are physically and mechanically removed by a knownmethod such as wet blasting, sandblasting, or ultrasonic cleaning.

However, where the constraining layers are removed from a sintered baselayer by wet blasting or sandblasting after the firing step as describedin Japanese Unexamined Patent Application Publication No. 2002-198646,the sintered base layer is damaged and the strength of the sintered baselayer is decreased, which may cause cracks. Furthermore, an electrodedisposed on a surface of the sintered base layer may be damaged. Thisreduces the strength of the electrode due to a decrease in the thicknessof the electrode that is rubbed off or reduces the connectivity betweenthe electrode and another component due to a decrease in the smoothnessof an electrode surface. Consequently, the reliability of the ceramicsubstrate produced by the method is reduced.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a method for efficiently manufacturing aceramic substrate with high reliability and a ceramic substrate withhigh reliability manufactured by the method. According to the method,when a ceramic substrate is manufactured through a constraint firingstep, a constraining layer is removed from a sintered base layer withoutcausing significant damage to, for example, the sintered base layer andan electrode provided on the surface of the sintered base layer, and thesintered base layer and the electrode can be effectively and reliablyexposed.

A method for manufacturing a ceramic substrate according to a preferredembodiment of the present invention includes a first step of forming agreen stacked body including a base layer that defines a ceramicsubstrate after firing, the base layer including ceramic powder andglass material, and a constraining layer arranged so as to be in contactwith at least one principal surface of the base layer, the constraininglayer being primarily composed of ceramic powder that is notsubstantially sintered at a sintering temperature of the base layer, asecond step of obtaining a fired stacked body having a sintered baselayer and a green constraining layer by firing the green stacked body tosinter the base layer, and a third step of removing the constraininglayer from the sintered base layer by vibrating media that are arrangedso as to be in contact with the constraining layer.

In the third step, an ultrasonic wave is preferably applied to vibratethe media while the fired stacked body and the media are immersed in aprocessing solution.

In the third step, the ultrasonic wave is preferably applied while thefired stacked body is disposed in a tray, with the media being spreadover the tray, such that the constraining layer is in contact with themedia.

In the third step, the fired stacked body is preferably disposed in atray such that the constraining layer faces upward, the media are spreadover the fired stacked body, and the ultrasonic wave is applied whilethe constraining layer is in contact with the media.

When a constraining layer of the fired stacked body is disposed on bothof one principal surface side and another principal surface side of thesintered base layer, in the third step, the fired stacked body ispreferably disposed in a tray, with the media being spread over thetray, the media are further spread over the fired stacked body, and theultrasonic wave is applied while the constraining layer disposed on bothof the one principal surface side and the other principal surface sideof the fired stacked body is in contact with the media.

Deaerated water is preferably used as the processing solution, forexample.

The specific gravity of the media is preferably greater than that of theprocessing solution.

The hardness of the media is preferably greater than that of theconstraining layer.

The media preferably have a substantially spherical shape, for example.

The media are preferably made of zirconia, for example.

A method for manufacturing a ceramic substrate according to a preferredembodiment of the present invention includes a first step of forming agreen stacked body including a base layer that defines a ceramicsubstrate after firing, the base layer including ceramic powder andglass material, an electrode being formed on a surface of the baselayer, and a constraining layer disposed so as to be in contact with atleast one principal surface of the base layer, the constraining layerbeing primarily composed of ceramic powder that is not sintered at asintering temperature of the base layer, a second step of obtaining afired stacked body having a reaction layer formed between a sinteredbase layer and the constraining layer by a reaction therebetween byfiring the green stacked body to sinter the base layer, and a third stepof removing the constraining layer from the sintered base layer byvibrating media that are disposed so as to be in contact with theconstraining layer, wherein, in the third step, at least a portion ofthe electrode is exposed while the reaction layer is remains at aboundary between a periphery of the electrode and the sintered baselayer around the electrode.

The reaction layer is preferably a layer including a ceramic componentincluded in the constraining layer and a glass component included in thebase layer. Specifically, the reaction layer is preferably formed withthe following mechanism. The glass component included in the base layerdiffuses into the constraining layer and the ceramic component includedin the constraining layer is then fixed with the glass component.Alternatively, the ceramic component included in the constraining layerand the glass component that has diffused into the constraining layerfrom the base layer are mixed together at the atomic level.

The reaction layer preferably remains not only at the boundary betweenthe periphery of the electrode and the sintered base layer around theelectrode but also in a different region of a surface of the sinteredbase layer.

A ceramic substrate according to a preferred embodiment of the presentinvention is manufactured by the method for manufacturing a ceramicsubstrate according to a preferred embodiment of the present inventiondescribed above.

In the method for manufacturing a ceramic substrate according to apreferred embodiment of the present invention, after a fired stackedbody having a sintered base layer and a green constraining layer is madeby firing a green stacked body to sinter a base layer, the constraininglayer is removed from the sintered base layer by vibrating media thatare disposed so as to be in contact with the constraining layer. Thus,the constraining layer is removed without causing damage to, forexample, the sintered base layer and an electrode disposed on thesurface of the sintered base layer, and the sintered base layer and theelectrode can be effectively and reliably exposed. Consequently, aceramic substrate with high reliability can be efficiently manufactured.

In various preferred embodiments of the present invention, theconstraining layer is removed at a portion at which media vibrates, and,for example, the constraining layer and a reaction layer formed by areaction between the constraining layer and the glass component includedin the base layer can be efficiently removed without causing damage tothe sintered base layer by suitably choosing the magnitude of thevibration or the shape and size of the media. Where an electrode isdisposed on a surface of the sintered base layer, a ceramic substratehaving excellent smoothness can be efficiently manufactured byselectively breaking a protruding reaction layer that is easily formedat the periphery of the electrode.

In various preferred embodiments of the present invention, a step ofpreliminarily removing an easily removable portion of the greenconstraining layer from the sintered base layer using a hand or a simplejig, such as a brush, that is, a constraining-layer preliminary removalstep can preferably be performed between the second step of firing thegreen stacked body and the third step of removing the constraining layerfrom the sintered base layer.

The phrase “media that are disposed so as to be in contact with theconstraining layer” includes the following concepts. For example, whenthe media are disposed on the constraining layer, the media momentarilyrise from the constraining layer due to the vibration, but fall down atthe next moment and thus the media become disposed on the constraininglayer again. Alternatively, when the fired stacked body is disposed onthe media such that the surface of the fired stacked body on which theconstraining layer is disposed faces downward, even if the constraininglayer is momentarily not in contact with the media due to the vibrationof the media, at the next moment the constraining layer is in contactwith the media again.

When, in the third step, an ultrasonic wave is applied to vibrate themedia while the fired stacked body and the media are immersed in aprocessing solution, the media are efficiently vibrated and thevibrational state of the media can be precisely controlled by using theultrasonic wave. Furthermore, by shifting the removed constraining layerto the processing solution, the reattachment of the constraining layerto the sintered base layer can be prevented.

When, in the third step, the ultrasonic wave is applied while the firedstacked body is disposed in a tray, with the media being spread over thetray, such that the constraining layer is in contact with the media, theconstraining layer can be extremely efficiently removed from the baselayer by vibrating the media while the constraining layer is closely incontact with the media.

In other words, since the media vibrate, the constraining layer issometimes not in contact with the media from a microscopic viewpoint,but the state in which the constraining layer is almost continuously incontact with the media can be maintained. Thus, the constraining layercan be efficiently removed.

When the constraining layer is disposed on both of one principal surfaceside and another principal surface side of the sintered base layer, inthe third step, the fired stacked body is disposed in a tray, with themedia being spread over the tray, the media are further spread over thefired stacked body, and the ultrasonic wave is applied while theconstraining layer disposed on both of the one principal surface sideand the other principal surface side of the fired stacked body are incontact with the media, whereby the state in which the constraininglayer is almost continuously in contact with the media can bemaintained. By vibrating the media in this state, the constraining layercan be quickly removed from both surfaces of the sintered base layer.

By using deaerated water as the processing solution, the sound pressureof the ultrasonic wave is increased, and thus, the vibration of themedia is increased. Therefore, the removal efficiency of theconstraining layer can be further improved.

When the specific gravity of the media is greater than that of theprocessing solution, the media do not float in the processing solutionand the media can be reliably located on the constraining layer. Thus,the constraining layer can be efficiently removed.

When the hardness of the media is greater than that of the constraininglayer, the constraining layer can be efficiently removed while thebrittle constraining layer are broken (ground).

When the media have a substantially spherical shape, the constraininglayer can be efficiently removed because the media easily rotate andmove on the constraining layer. Moreover, a ceramic substrate havinggood surface condition can be efficiently manufactured because suchmedia do not easily cause damage to the sintered base layer.

When the media are made of zirconia, for example, the constraining layercan be efficiently removed because zirconia has a specific gravity ofabout 5.8, which is greater than that of a solution normally used forthe processing solution, and zirconia also has a high Mohs hardness ofabout 8.5.

A reaction layer formed by a reaction between a glass component includedin the base layer and a material of the constraining layer is easilyformed at a portion of the constraining layer after firing that is closeto the boundary between the base layer and the constraining layer. Sincethe reaction layer has high adhesion with the base layer and is noteasily removed, the removal of the reaction layer is often problematicwhen the constraining layer is removed. However, the hardness of thereaction layer formed by a reaction with a glass component normallytends to be less than that of the material itself of the constraininglayer (e.g., alumina). Therefore, when the media are made of zirconia,the hardness of the media is often greater than that of the reactionlayer. Thus, the reaction layer can be efficiently removed.

Where, after a fired stacked body having a reaction layer formed betweena sintered base layer and a constraining layer with a reactiontherebetween is made by firing a green stacked body, at least a portionof the electrode is exposed by vibrating media that are disposed so asto be in contact with the constraining layer while the reaction layer isleft at the boundary between the periphery of the electrode and thesintered base layer around the electrode as in another method formanufacturing a ceramic substrate according to a preferred embodiment ofthe present invention, the reaction layer can preferably be used toreduce the difference in elevation between the surfaces of the electrodeand the sintered base layer. Thus, a ceramic substrate having excellentcoplanarity due to a small difference in elevation between the surfacesof the electrode and the sintered base layer can preferably bemanufactured. Furthermore, the reaction layer preferably remains so asto cover the boundary between the periphery of the electrode and thesintered base layer around the electrode, whereby the reaction layerfunctions as a protective layer that prevents and minimizes themigration of electrode material.

In other words, sandblasting and wet blasting are methods for grindingthe layer or material to be removed or the reaction layer using anabrasive grain whereas the method of preferred embodiments of thepresent invention is a method for rubbing off constraining layer grainsusing the media. Since the vibrational energy of the media is not overlylarge, the constraining layers are removed, but the close-grainedreaction layer is not rubbed off and most of the reaction layer canremain by adjusting, for example, the magnitude of the vibration of themedia. Consequently, the reaction layer that normally remains on thesurface of the sintered base layer (ceramic substrate) with a thicknessof several micrometers to several tens of micrometers decreases thedifference in elevation between the surfaces of the electrode and thesintered base layer. This can significantly improve coplanarity.

The reaction layer formed at the boundary between the constraining layerand the base layer by a reaction between a glass component included inthe base layer and a material of the constraining layer may protrude,for example, by about several micrometers on the electrode. By adjustingthe magnitude of the vibration of the media in accordance with theprotruding reaction layer, the protruding portion is selectively removedsuch that the reaction layer has a gentle slope.

In preferred embodiments of the present invention, a step ofpreliminarily removing an easily removable portion of the greenconstraining layer from the sintered base layer using a hand or a simplejig such as a brush, that is, a constraining-layer preliminary removalstep can preferably be included between the second step of firing thegreen stacked body and the third step of removing the constraining layerfrom the sintered base layer.

By leaving the reaction layer not only at the boundary between theperiphery of the electrode and the sintered base layer around theelectrode but also in a different region of a surface of the sinteredbase layer, the difference in elevation between the surfaces of theentire sintered base layer and the electrode is decreased and thecoplanarity can be further improved.

A ceramic substrate according to a preferred embodiment of the presentinvention is manufactured by the method for manufacturing a ceramicsubstrate according to a preferred embodiment of the present inventiondescribed above. The constraining layer is removed while the damage to,for example, the sintered base layer (ceramic substrate) and anelectrode provided on a surface of the sintered base layer areprevented, and the sintered base layer and the electrode can be reliablyexposed. Thus, a ceramic substrate having excellent characteristics canpreferably be provided.

Where an electrode is disposed on a surface of the sintered base layerand a reaction layer formed by a reaction between the constraining layerand the base layer in the firing step remains at the boundary betweenthe periphery of the electrode and the sintered base layer around theelectrode, a ceramic substrate having excellent characteristics andcoplanarity due to a small difference in elevation between the surfacesof the electrode and the base layer can be provided.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a green stacked body that includes constraining layersdisposed on upper and lower surface sides of a base layer and that ismanufactured in Example 1 of a preferred embodiment of the presentinvention, and FIG. 1B shows a fired stacked body obtained by firing thegreen stacked body of FIG. 1A.

FIG. 2 shows a method for removing the constraining layers from thefired stacked body obtained by firing the green stacked body of FIG. 1A.

FIG. 3 shows an example of a ceramic multilayer substrate according to apreferred embodiment of the present invention that can be manufacturedusing a method according to a preferred embodiment of the presentinvention.

FIG. 4 shows a green stacked body that includes constraining layersdisposed on both upper and lower surface sides of a base layer and thatis manufactured in an Example 3 of a preferred embodiment of the presentinvention.

FIG. 5 shows a fired stacked body obtained by firing the green stackedbody of FIG. 4.

FIG. 6 shows a method for removing the constraining layers from thefired stacked body obtained by firing the green stacked body in Example3.

FIG. 7A shows a principal portion of a ceramic substrate manufactured bythe method of Example 3 before a plating film is formed on a surfaceelectrode, and FIG. 7B shows a principal portion of a ceramic substratemanufactured by the method of Example 3 after a plating film is formedon a surface electrode.

FIG. 8 shows a difference in elevation between the surface electrode andthe surface of a sintered base layer in a ceramic substrate in which aconstraining layer is removed by the method of a Comparative Example.

FIG. 9 shows a method for manufacturing a ceramic substrate by anexisting constraint firing method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Examples of preferred embodiments of the present invention will bedescribed with reference to the drawings.

Example 1

First, a base layer defining a principal portion of a ceramic substrateafter firing was made by the following method.

Alumina powder was prepared as ceramic powder, and borosilicate glasspowder including about 59% by weight of SiO₂, about 10% by weight ofB₂O₃, about 25% by weight of CaO, and about 6% by weight of Al₂O₃, forexample, was prepared as glass powder.

The alumina powder and the glass powder were preferably mixed, forexample, in a ratio of about 40:60 by weight. Proper amounts of abinder, a dispersant, a plasticizer, an organic solvent, and othercomponents were added to the resultant mixed powder and they were mixedto make ceramic slurry.

The ceramic slurry was then formed into a sheet by a method such as adoctor blade method, for example, to make a base-layer green sheet.

A base layer A was formed by stacking a plurality of base-layer greensheets. To evaluate characteristics, a base-layer ceramic green sheet onwhich an interdigital electrode (for evaluating reliability) 10 with aline width of about 0.1 mm and a gap of about 0.1 mm, for example wasformed was prepared as a base-layer green sheet that defines the uppersurface of the base layer A. The interdigital electrode 10 waspreferably formed by screen printing Ag electrode paste, for example.

A ceramic green sheet to define a constraining layer was made asfollows.

A ceramic powder that is not substantially sintered at the firingtemperature of the base-layer ceramic green sheet, that is, preferablyAl₂O₃ powder having an average particle size of about 1.0 μm, forexample, in Example 1 was dispersed into an organic vehicle composed ofan organic binder, an organic solvent, a plasticizer, and othercomponents to prepare a slurry.

The obtained slurry was formed into a sheet to make a constraining-layerceramic green sheet preferably having a thickness of about 300 μm, forexample.

This constraining-layer ceramic green sheet preferably has a sinteringtemperature of about 1400° C. to about 1600° C., for example, and isthus not substantially sintered at the sintering temperature of thebase-layer ceramic green sheet.

As shown in FIG. 1A, a constraining-layer ceramic green sheet 2 (2 a), aplurality of base-layer ceramic green sheets 1, and a constraining-layerceramic green sheet 2 (2 b) were then stacked in that sequence.Subsequently, a green stacked body 11 having a structure in whichconstraining layers (2 a and 2 b) were disposed on both upper and lowersurface sides of the base layer (green ceramic substrate) A was made bypressure bonding at a pressure of, for example, about 5 MPa to about 200MPa using isostatic pressing or other suitable method (refer to FIG.1A).

In Example 1, the plurality of base-layer ceramic green sheets 1 werestacked such that the thickness of the base layer (green ceramicsubstrate) A is preferably about 300 μm, for example. As describedabove, the base-layer ceramic green sheet 1 (1 a) on which theinterdigital electrode (Ag electrode) 10 for evaluating reliability wasformed was used as a base-layer ceramic green sheet 1 that defines theupper surface of the base layer A.

A single constraining-layer ceramic green sheet preferably having athickness of about 300 μm, for example, was stacked on each of the upperand lower surface sides of the base layer (green ceramic substrate) A toform the constraining layers 2 (2 a and 2 b) having a thickness of about300 μm.

Although the base layer A having a multilayer structure was made bystacking the plurality of base-layer ceramic green sheets 1 in Example1, a base layer having a single layer structure may be made using asingle base-layer ceramic green sheet 1 to manufacture a single-plateceramic substrate.

Although the constraining layers 2 were disposed on both of the upperand lower surface sides of the base layer A in Example 1, theconstraining layer 2 may be disposed on only one principal surface ofthe base layer A.

In Example 1, each of the constraining layers 2 was defined by a singleconstraining-layer ceramic green sheet. However, each constraining layer2 may be formed by stacking a plurality of constraining-layer ceramicgreen sheets.

The green stacked body 11 was then heat-treated at a low debindingtemperature (e.g., about 400° C.) in the atmosphere to remove an organicsubstance such as a binder.

After that, the temperature was increased to about 900° C., for example,such that the base layer is sintered but the ceramic powder defining theconstraining layers is not sintered, to fire the green stacked body 11.Thus, as shown in FIG. 1B, a fired stacked body 21 with greenconstraining layers 2 (2 a and 2 b) disposed on both principal surfacesof a sintered base layer A_(F) was obtained.

The constraining layers 2 (2 a and 2 b) were removed from the firedstacked body 21 obtained as described above by the following method.

As shown in FIG. 2, spherical media (hereinafter also referred to aszirconia balls) 12 preferably having a diameter of about 2 mm and madeof zirconia (ZrO₂), for example, were spread over a stainless steel tray13, and the fired stacked body 21 with green constraining layers 2 (FIG.1B) disposed on both principal surfaces of the sintered base layer A_(F)(FIG. 1B) was disposed thereon. Furthermore, for example, at least asingle layer of zirconia balls 12 were disposed on the fired stackedbody 21 (a single layer of zirconia balls 12 is provided in FIG. 2).

All of the zirconia balls 12, the tray 13, and the fired stacked body 21were inserted into an ultrasonic cleaning tank (output about 600 W,frequency about 40 KHz) 15 filled with a processing solution 14. Aconstraining-layer removal process was performed for about 30 minutes byapplying an ultrasonic wave using an ultrasonic oscillator 16 to vibratethe zirconia balls 12.

To increase sound pressure when the ultrasonic wave is applied,deaerated water was preferably used as the processing solution 14.

For comparison, the constraining layers were removed from a firedstacked body with green constraining layers disposed on both principalsurfaces of the sintered base layer that was made by the same method asthat of Example 1, by spraying slurry including about 15% of a about#500 alumina abrasive grain at about 0.15 MPa through wet blasting.

After the sintered base layers from which the constraining layers wereremoved by the methods of Example 1 and the Comparative Example werecleaned, a Ni plating film having a thickness of about 5 μm, forexample, was formed on the interdigital electrode (Ag electrode) that isdisposed on the surface of each of the sintered base layers.

Subsequently, a Pd plating film having a thickness of about 0.2 μm, forexample, was formed on the Ni plating film, and an Au plating filmhaving a thickness of about 0.1 μm, for example, was formed on the Pdplating film. As a result, an electrode with a plating film having athree-layer structure was formed on the Ag electrode.

For the sample obtained by removing the constraining layers using themethod of Example 1 and the sample obtained by removing the constraininglayers using the method of Comparative Example, the surface roughness Raof the electrodes (interdigital electrodes), the wire-pull strength (W/Bpull strength) of the electrodes, and the flexural strength weremeasured while a reliability test was conducted to evaluate reliability.The results are shown in Table 1.

TABLE 1 Comparative Example Example (N = 10) (N = 10) Surface roughness0.15 0.60 Ra (μm) Wire-pull strength 8.05 6.45 (average) (gf) Flexuralstrength 325 295 (average) (MPa) Reliability test Good Poor

The surface roughness Ra of the electrodes, the wire-pull strength, andthe flexural strength were measured by the methods described below, andthe reliability test was conducted by the method described below.

(1) Surface Roughness Ra

The surface roughness Ra was obtained by measuring the line roughness ofthe interdigital electrode using a laser microscope.

(2) Wire-Pull Strength

An Au wire with a diameter of about 20 μm and a length of about 800 μmwas connected to the interdigital electrode, and the wire was pulledusing a wire-pull tester. The tensile force when the wire was cut orwhen the connecting portion and its vicinity were broken or peeled offwas defined as the wire-pull strength.

(3) Flexural Strength

A test piece having a size of L (length)×W (width)×T (thickness)=about30 mm×about 4 mm×about 0.3 mm was processed with a three-point bendingtester. The load when the test piece was broken was defined as theflexural strength.

(4) Reliability Test

A voltage of about 20 V was applied to a ceramic substrate on which aninterdigital electrode with line/space=about 100 μm/about 100 μm wasformed, for about 120 hours in an atmosphere of about 85° C. and about85% RH. When insulation resistance (IR) degradation and Ag migrationwere not caused, the result was defined as good. When either IRdegradation or Ag migration was caused, the result was defined as poor.

As shown in Table 1, the surface roughness Ra of the electrode was about0.60 μm, which was relatively high, for the sample of the ComparativeExample. In contrast, the surface roughness Ra of the electrode wasabout 0.15 μm, which was relatively low, for the sample of the Example.It was confirmed that the sample of the Example had significantly bettersmoothness than that of the Comparative Example.

It was confirmed that the wire-pull strength of about 8.05 gf of thesample of the Example was greater than the wire-pull strength of about6.45 gf of the sample of the Comparative Example.

It was confirmed that the flexural strength of about 325 MPa of thesample of the Example was greater than the flexural strength of about295 MPa of the sample of the Comparative Example.

It was also confirmed that the sample of the Comparative Example hadpoor reliability in the reliability test whereas the sample of theExample had good reliability.

It was confirmed from the results described above that, according to themethod of Example 1 in which a fired stacked body having a sintered baselayer and green constraining layers is made and the constraining layersare then removed from the sintered base layer by applying an ultrasonicwave to vibrate media that are disposed so as to be in contact with theconstraining layers, the constraining layers can be efficiently removedand the surface smoothness of the sintered base layer can be improvedwithout causing damage to the ceramic surface defining the sintered baselayer or the electrode formed on the surface of the base layer, forexample.

According to the knowledge of the inventors, in the method of theExample described above, the media momentarily rise from theconstraining layers due to their vibration, but the media almostcontinuously remain on the constraining layers from a macroscopicviewpoint. Thus, the energy provided to the fired stacked body when themedia settle can be decreased. Accordingly, damage to the ceramicsurface or the electrode can be suppressed as much as possible.

In Example 1, spherical zirconia balls having a diameter of about 2 mm,for example, are preferably used as the media and the constraininglayers are removed by applying an ultrasonic wave to vibrate the media.Therefore, when the constraining layers on the electrode disposed on thesurface of the sintered base layer are removed, an effect in which theelectrode is rolled by the media is produced after the constraininglayer grains on the electrode are removed with the media, which lowerssurface roughness. Such low surface roughness of the electrode improvesthe wire-pull strength, which is well known.

Furthermore, a grain boundary is not readily formed in an Au platingfilm disposed on the electrode surface due to low surface roughness, anda phenomenon in which Ni migrates to the surface through the grainboundary during heat treatment is prevented. It is believed that thisalso contributes to the improved wire-pull strength.

In the method of Example 1, the energy provided to the ceramic substratefrom vibrating media is preferably sufficient to remove a reaction layerformed between the sintered base layer and the constraining layers, butis not sufficient to grind the sintered base layer (the substrateitself). Thus, the substrate itself is not significantly ground and thedamage to the surface of the substrate can be decreased as compared tothe Comparative Example. As a result, minute flaws that could cause thedestruction of the substrate itself are prevented from developing, andthe flexural strength can be improved as compared to the ComparativeExample (refer to Table 1). In other words, high flexural strength isachieved in the method of Example 1 because minute flaws that couldcause the destruction of the substrate itself do not readily develop andthe minimum value of the flexural strength is increased while thevariation thereof is decreased rather than the strength of the substrateitself being improved, whereby an average of flexural strength isincreased.

Although not shown in Table 1, the ceramic substrate manufactured by themethod of Example 1 preferably had a thickness of about 0.3 mm asdesigned, whereas the ceramic substrate manufactured by the method ofthe Comparative Example had a thickness of about 0.285 mm. In theComparative Example, the thickness of the ceramic substrate is decreasedby about 0.015 mm for both principal surfaces, which may cause thedecreased flexural strength.

Even where the method of Example 1 is applied to the manufacture of aceramic multilayer substrate having a common structure, that is, aceramic multilayer substrate B that includes inner conductors 32disposed between a plurality of stacked insulating ceramic layers 31 anda surface conductor 34 formed on the surface of a stacked body 33 andthat has a structure in which the inner conductors 32 are connected toeach other or each of the inner conductors 32 and the surface conductor34 are connected through a via hole conductor 35 as shown in FIG. 3, thesame advantages as those described with respect to in Example 1 areachieved.

Example 2

A ceramic substrate was manufactured by the step of removing theconstraining layers from the sintered stacked body after firing in thesame method as that of Example 1, except that spherical media (zirconiaballs) having a diameter of about 1 mm and made of zirconia (ZrO₂) andspherical zirconia balls having a diameter of about 3 mm were used.

Consequently, it was confirmed that the removal of the constraininglayers becomes more difficult when the zirconia balls having a diameterof about 1 mm were used as media.

This is because, when the zirconia balls having a diameter of about 1 mmwere used, sufficient energy to break a reaction layer formed betweenthe sintered base layer and the constraining layers was not provided dueto their mass being relatively small even if the media were vibrated byapplying an ultrasonic wave. When the output of the ultrasonic wave wasincreased to provide higher energy (that is, the sound pressure wasincreased), the media flew out of the tray due to their mass being toosmall. As a result, a desired effect was not produced.

When the zirconia balls having a diameter of about 3 mm were used asmedia, it was confirmed that a region in which the constraining layerswere not removed partially remained and the removal of the constraininglayers became insufficient.

This is because, when the zirconia balls having a diameter of about 3 mmwere used, sufficient energy was provided due to their mass beingrelatively large, but the intervals between contact points becameexcessively large due to their curvature being too large, which causeduneven removal of the constraining layers. To completely remove theconstraining layers by uniformly rubbing off the entire surface of thesubstrate with the zirconia balls having a diameter of about 3 mm, along period of time is required to conduct the constraining-layerremoval step, which decreases productivity.

Accordingly, spherical zirconia balls having a diameter in a range ofabout 1 mm about 3 mm, for example, are preferably used under theconditions described in Example 1.

However, since the preferable diameter range of the media is affected bythe thickness of the constraining layer, the material of theconstraining layer, the specific gravity of the processing solution, orother factors, the preferable diameter range of the media is notnecessarily limited to the range described above.

Example 3

First, a base layer defining a principal portion of a ceramic substrateafter firing was made by the following method.

Alumina powder was prepared as ceramic powder, and borosilicate glasspowder including about 59% by weight of SiO₂, about 10% by weight ofB₂O₃, about 25% by weight of CaO, and about 6% by weight of Al₂O₃, forexample, was also prepared as glass powder.

The alumina powder and the glass powder were preferably mixed, forexample, in a ratio of about 40:60 by weight. Proper amounts of abinder, a dispersant, a plasticizer, an organic solvent, and othercomponents were added to the resultant mixed powder and they were mixedto make ceramic slurry.

The ceramic slurry was then formed into a sheet by a method such as adoctor blade method to make a base-layer green sheet.

Subsequently, a through hole 102 (FIG. 4) arranged to form a via holeconductor was preferably formed in the obtained base-layer ceramic greensheet 101 a. The through hole 102 was preferably filled with conductivepaste or conductive powder to form a green via hole conductor 123 a(FIG. 4). In Example 3, the through hole 102 was preferably filled withconductive paste composed of Ag, for example, that is a conductivecomponent.

A green surface electrode 121 a and inner conductors 122 a werepreferably formed on the base-layer ceramic green sheet 101 a byprinting, for example, Ag conductive paste (refer to FIG. 4).

A constraining-layer ceramic green sheet was made as follows. A ceramicpowder (alumina powder in Example 3) that is not substantially sinteredat the firing temperature of the base-layer ceramic green sheet wasdispersed into an organic vehicle composed of an organic binder, anorganic solvent, a plasticizer, and other components to prepare aslurry.

The obtained slurry was formed into a sheet to make a constraining-layerceramic green sheet.

In Example 3, the thickness of the constraining-layer ceramic greensheet was preferably set to be about 300 μm, for example, to achievesufficient constraining force.

As shown in FIG. 4, a constraining layer 131, a plurality of base-layerceramic green sheets 101 a, and a constraining layer (constraining-layergreen sheet) 131 were then stacked in that sequence. Subsequently, agreen stacked body 132 having a structure in which the constraininglayers 131 were preferably disposed on both upper and lower surfacesides of a green base layer (green ceramic substrate) A was made bypressure bonding (refer to FIG. 4).

In Example 3, the thickness of the green base layer (green ceramicsubstrate) A was preferably set to be about 300 μm, for example, and thethickness of the constraining layers 131 disposed on both upper andlower principal surfaces of the green base layer A was preferably set tobe about 300 μm.

Although the base layer A having a multilayer structure was made bystacking the plurality of base-layer ceramic green sheets 101 a inExample 3, a base layer having a single layer structure may preferablybe made using a single base-layer ceramic green sheet 101 a tomanufacture a single-plate ceramic substrate.

Although the constraining layers 131 were disposed on both upper andlower surface sides of the base layer A in Example 3, the constraininglayer 131 may preferably be disposed on only one principal surface ofthe base layer A.

In Example 3, each of the constraining layers 131 was formed from asingle constraining-layer ceramic green sheet, but may preferably beformed by stacking a plurality of constraining-layer ceramic greensheets.

The green stacked body 132 was then heat-treated at a low debindingtemperature (e.g., about 400° C.) in the atmosphere to remove an organicsubstance such as a binder.

After that, the temperature was increased to about 900° C., for example,under the conditions under which the base layer A is sintered but theceramic powder defining the constraining layers 131 is not sintered, tofire the green stacked body 132. Thus, as shown in FIG. 5, a firedstacked body 133 with green constraining layers 131 disposed on bothprincipal surfaces of a sintered base layer A_(F) was obtained. Thesintered base layer A_(F) preferably defines a ceramic substrate(ceramic multilayer substrate) after the constraining layers 131 areremoved. The sintered base layer A_(F) includes an electrode (surfaceelectrode) 121 formed on the surface thereof and inner conductors 122disposed between a plurality of stacked insulating ceramic layers 101and has a structure in which the inner conductors 122 are connected toeach other or each of the inner conductors 122 and the surface electrode121 are connected through a via hole conductor 123.

The constraining layers 131 were removed from the thus-obtained firedstacked body 133 while a reaction layer 124 (FIGS. 7A and 8) formedbetween the sintered base layer A_(F) and the constraining layers 131with the reaction therebetween preferably remained, by the methoddescribed below.

As shown in FIG. 6, spherical media (hereinafter also referred to aszirconia balls) 112 preferably having a diameter of about 2 mm and madeof zirconia (ZrO₂), for example, were spread over a stainless steel tray113, and the fired stacked body 133 with green constraining layers 131(FIG. 5) disposed on both principal surfaces of the sintered base layerA_(F) (FIG. 5) was disposed thereon. Furthermore, for example, at leasta single layer of zirconia balls 112 as media were preferably disposedon the fired stacked body 133 (a single layer of zirconia balls 112 isplaced in FIG. 6).

All of the zirconia balls 112, the tray 113, and the fired stacked body133 were inserted into an ultrasonic cleaning tank 115 filled with aprocessing solution 114. A constraining-layer removal process waspreferably performed for about 15 minutes, for example, by applying anultrasonic wave using an ultrasonic oscillator (output about 600 W,frequency about 40 KHz) 116 to vibrate the zirconia balls 112.

In this case, the constraining layers 131 were removed such that atleast a portion of the surface electrode 121 formed on the surface ofthe sintered base layer A_(F) was exposed while the reaction layer 124remained in the region from the periphery of the surface electrode 121to the surface of the sintered base layer (ceramic substrate) A_(F).

To increase sound pressure when the ultrasonic wave is applied,deaerated water, for example, was preferably used as the processingsolution 114.

For comparison, the constraining layers were removed from the firedstacked body with green constraining layers disposed on both principalsurfaces of the sintered base layer that was made by the same method asthat of Example 3, by spraying slurry including about 15% of a about#500 alumina abrasive grain at about 0.15 MPa through wet blasting.

After the sintered base layers from which the constraining layers wereremoved by the methods of Example 3 and the Comparative Example werecleaned, a Ni plating film preferably having a thickness of about 5 μm,for example, was formed on the surface electrode.

Subsequently, a Pd plating film preferably having a thickness of about0.2 μm, for example, was formed on the Ni plating film, and an Auplating film preferably having a thickness of about 0.1 μm, for example,was formed on the Pd plating film. As a result, an electrode (surfaceelectrode) 121 with a plating film 141 (refer to FIGS. 7B and 8) havinga three-layer structure was formed on the Ag electrode.

For the samples made by the methods of Example 3 and the ComparativeExample, the difference in elevation ΔH between the surface electrodehaving the plating film and the sintered base layer was measured. Theresult is shown in Table 2.

TABLE 2 Difference in elevation between surface electrode and surface ofceramic substrate (μm) Example 3 Comparative Example 12

The difference in elevation ΔH was obtained by measuring projections anddepressions of a surface using Laser Microscope VK9700 available fromKEYENCE CORPORATION.

As shown in Table 2, the difference in elevation ΔH between the surfaceelectrode 121 and the sintered base layer (ceramic substrate) A_(F) wasabout 12 μm for the sample of the Comparative Example, whereas thedifference in elevation ΔH was about 3 μm for the sample of Example 3.That is to say, the difference in elevation for the sample of Example 3was significantly reduced to about 3 μm due to the following reason.

Where a fired stacked body having a sintered base layer and greenconstraining layers is made and a constraining-layer removal process isthen conducted for about 15 minutes, for example, by vibrating mediathat are disposed so as to be in contact with the constraining layers asin Example 3, as schematically shown in FIG. 7A, the surface electrode121 formed on the surface of the sintered base layer (ceramic substrate)A_(F) is exposed while the reaction layer 124 formed by the reactionbetween the base layer and the constraining layers remains in the regionfrom the periphery of the surface electrode 121 to the surface of thesintered base layer A_(F).

As a result, as schematically shown in FIG. 7B, even when the platingfilm 141 described above is formed on the surface of the surfaceelectrode 121, the reaction layer 124 functions to decrease thedifference in elevation ΔH between the surface electrode 121 includingthe plating film 141 and the surface of the ceramic substrate A_(F).Thus, a large difference in elevation ΔH is prevented.

In other words, the method according to a preferred embodiment of thepresent invention is to rub off constraining layer grains on theelectrode using media and the vibrational energy of the media is notoverly large. By suitably adjusting the conditions of theconstraining-layer removal process (e.g., processing time, the size ofmedia, and the magnitude of power applied), the constraining layers aresufficiently removed, but the close-grained reaction layer is notsignificantly rubbed off and most of the reaction layer remains.Consequently, since the height of the surface of the surface electrodedoes not change even after media are vibrated due to the reaction layerthat normally remains on the surface of the ceramic substrate (sinteredbase layer) with a thickness of several micrometers to several tens ofmicrometers, the difference in elevation between the surfaces of thesurface electrode and the ceramic substrate is decreased, whichsignificantly improve coplanarity.

In contrast, the method of the Comparative Example is to grind theconstraining layers and the reaction layer using an abrasive grain andthe reaction layer that is harder than the constraining layers isquickly ground, which readily a level difference. When the constraininglayers on the electrode have been removed, almost all the reaction layerformed by the reaction between the base layer and the constraininglayers has also has been removed. Therefore, as schematically shown inFIG. 8, the reaction layer does not decrease the difference in elevationΔH between the surface electrode 121 including the plating film 141 andthe sintered base layer (ceramic substrate) A_(F), and thus such a largevalue shown in Table 2 is obtained.

In the method of Example 3, the constraining layers 131 can preferablybe removed such that at least a portion of the electrode (surfaceelectrode) formed on the surface of the sintered base layer A_(F) isexposed while the reaction layer remains at the boundary between theperiphery of the surface electrode and the sintered base layer aroundthe surface electrode. The reaction layer decreases the difference inelevation between the surfaces of the surface electrode and the ceramicsubstrate, which significantly improves coplanarity.

In Example 3, the reaction layer preferably remains so as to cover theboundary between the periphery of the exposed surface electrode and thesintered base layer around the surface electrode. Thus, for example, thereaction layer functions as a protective layer that suppresses themigration of electrode material.

To measure the effects of the particle size of media, a ceramicsubstrate was manufactured using the step of removing the constraininglayers from the fired stacked body in the same method as that of Example3, except that spherical media (zirconia balls) having a diameter ofabout 1 mm and made of zirconia (ZrO₂) and spherical zirconia ballshaving a diameter of about 3 mm were used.

Consequently, when the zirconia balls having a diameter of about 1 mmwere used, the energy was too low to efficiently remove the constraininglayers due to their mass being too small even if the media were vibratedby applying an ultrasonic wave, which was undesirable.

When the output of the ultrasonic wave was increased to provide higherenergy (that is, sound pressure was increased), the media flew out ofthe tray due to their mass being too small. As a result, a desiredeffect was not produced.

When the zirconia balls having a diameter of about 3 mm were used,sufficient energy was provided due to their mass being relatively large,but the intervals between contact points became large due to theircurvature being too large, which prevented the constraining layers frombeing efficiently removed.

Accordingly, spherical zirconia balls having a diameter in a range ofabout 1 mm to about 3 mm, for example, are preferably used under theconditions described in Example 3.

However, since the preferable diameter range of the media is affected bythe thickness of the constraining layer, the material of theconstraining layer, the specific gravity of the processing solution, orother factors, the preferable diameter range of the media is notnecessarily limited to the range described above.

The present invention is not limited to the Examples of preferredembodiments described above. Various applications and modifications canbe made within the scope of the present invention regarding the specifickind or blending ratio of ceramic powder and glass material constitutingthe base layer, the structure or material of the electrode disposed inthe base layer, the specific kind of material constituting theconstraining layer, and the conditions under which an ultrasonic wave isapplied.

According to preferred embodiments of the present invention as describedabove, when a ceramic substrate is manufactured with a constraint firingstep, a constraining layer is removed from a sintered base layer withoutcausing significant damage to, for example, the sintered base layer andan electrode formed on the surface of the sintered base layer and theelectrode can be reliably exposed. Consequently, a ceramic substratewith high reliability can be efficiently manufactured.

Accordingly, preferred embodiments of the present invention can bewidely used for the manufacturing technology of a ceramic substratemanufactured through a constraint firing step.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A ceramic substrate produced by a method for manufacturing a ceramicsubstrate comprising: a first step of forming a green stacked bodyincluding: a base layer defining a ceramic substrate after firing, thebase layer including ceramic powder and glass material; an electrodedisposed on at least one principal surface of the base layer; and aconstraining layer disposed so as to be in contact with the at least oneprincipal surface of the base layer and the electrode disposed thereon,the constraining layer being primarily composed of ceramic powder thatis not substantially sintered at a sintering temperature of the baselayer; a second step of firing the green stacked body to obtain a firedstacked body having a sintered base layer, the constraining layer, and areaction layer formed by a reaction between the base layer and theconstraining layer during firing; and a third step of removing theconstraining layer from the sintered base layer by vibrating media thatare disposed so as to be in contact with the constraining layer; whereinat least a portion of the reaction layer remains after the third step ofremoving the constraining layer.
 2. The ceramic substrate produced bythe method for manufacturing a ceramic substrate according to claim 1,wherein, in the third step, at least a portion of the electrode isexposed while the reaction layer remains at a boundary between aperiphery of the electrode and a first region of a surface of thesintered base layer located around the electrode.
 3. The ceramicsubstrate produced by the method for manufacturing a ceramic substrateaccording to claim 2, further comprising a fourth step of forming aplating film on the electrode.
 4. The ceramic substrate produced by themethod for manufacturing a ceramic substrate according to claim 2,wherein the reaction layer also remains in a second region of thesurface of the sintered base layer that is different from the firstregion of the surface of the sintered base layer.
 5. The ceramicsubstrate produced by the method for manufacturing a ceramic substrateaccording to claim 1, wherein, in the third step, an ultrasonic wave isapplied to vibrate the media while the fired stacked body and the mediaare immersed in a processing solution.
 6. The ceramic substrate producedby the method for manufacturing a ceramic substrate according to claim5, wherein, in the third step, the ultrasonic wave is applied while thefired stacked body is disposed in a tray, with the media being arrangedin the tray, such that the constraining layer is in contact with themedia.
 7. The ceramic substrate produced by the method for manufacturinga ceramic substrate according to claim 5, wherein, in the third step,the fired stacked body is disposed in a tray such that the constraininglayer faces upward, the media are spread over the fired stacked body,and the ultrasonic wave is applied while the constraining layer is incontact with the media.
 8. The ceramic substrate produced by the methodfor manufacturing a ceramic substrate according to claim 5, wherein,when the constraining layer of the fired stacked body is disposed onboth of one principal surface side and another principal surface side ofthe sintered base layer, in the third step, the fired stacked body isdisposed in a tray, with the media being arranged in the tray, the mediaare further spread over the fired stacked body, and the ultrasonic waveis applied while the constraining layer disposed on both of the oneprincipal surface side and the other principal surface side of the firedstacked body is in contact with the media.
 9. The ceramic substrateproduced by the method for manufacturing a ceramic substrate accordingto claim 5, wherein deaerated water is used as the processing solution.10. The ceramic substrate produced by the method for manufacturing aceramic substrate according to claim 5, wherein a specific gravity ofthe media is greater than that of the processing solution.
 11. Theceramic substrate produced by the method for manufacturing a ceramicsubstrate according to claim 1, wherein a hardness of the media isgreater than that of the constraining layer.
 12. The ceramic substrateproduced by the method for manufacturing a ceramic substrate accordingto claim 1, wherein the media have a substantially spherical shape. 13.The ceramic substrate produced by the method for manufacturing a ceramicsubstrate according to claim 1, wherein the media are made of zirconia.